EWSR1 and Ewing sarcoma: These challenges are further appreciated when considering the various attempts to develop a transgenic mouse model of Ewing sarcoma: constitutively expressed EWS/FLI is embryonic lethal (Torchia et al., 2007), where inducible, tissue-specific expression of EWS/FLI in MPC resulted in limb bud developmental abnormalities (Lin et al., 2008).